The RDBMS has not fundamentally changed much since the 80's. It's the same basic design inside and out, with a shitload of tweaks and optimizations. Don't get me wrong - you can get very far with tweaks and optimizations. But the foundations root you to a particular model, and some things in that model will always be painful.
The important question to me isn't why don't people upgrade. It's why do people run Postgres?
Is there no other kind of database? No, there are hundreds of different databases. Is it because Postgres does things fundamentally different and better than anything else? No, lots of things work somewhat like Postgres, with pros and cons. Is it because you can't do things without Postgres? No, there are always alternatives that you can make work. Is it because it's impossible for it to keep working without upgrading? No, any database with long-term support could continue working without upgrading.
So why use Postgres at all? Answer: it's the incumbency, stupid.
When a big fucking honking giant "thing" sucks up all the air in the room, provides for most of the use cases, and is accepted by all the users, then it's extremely hard to justify not using it. Incumbents usually win, even if they're old, disreputable, buggy, and annoying. Even if they're missing some obvious features other things have. Even if people loudly complain that they want change. It's just more annoying not to use them.
We're used to them. We've adapted. Trying to do something else is going to be annoying and hard. So we stick to what we know. But that doesn't mean we have to stroke the balls while working the shaft. Upgrading isn't exactly fun. It doesn't even buy us anything, other than the promise of "support", or "compatibility" with whatever else will eventually require it. So we upgrade, eventually, when we must.
But the constant mind-numbing march away from entropy isn't a fantastic reason to keep using the thing. When you have to convince yourself to stick with it, it's likely you're in an toxic relationship. If you're honest with yourself, you'll agree that it's time for a change. You deserve better.
But switching is full of unknowns. Ominous. Time-consuming. Hard. You know you want to, maybe even need to. But there's too much to lose. So you stick to the familiar, to what you can handle. Maybe something better will come down the pike soon. You even tell yourself you're lucky to be here. But deep down you know you're just comforting yourself. You wish you had something else. Something that brings you joy. Something better. Something... right. But that something isn't here right now. So until Mr. Right gets here, you'll stick with Mr. Right Now.
Time to get ready for the next upgrade..... sigh...